Conventional nondestructive evaluation (NDE) techniques have been used in the Space Shuttle program to screen for defects (e.g., cracks, debond, voids, etc.) in the basic case, insulation, propellant assembly of the solid rocket motor. Uncertainty in the size, location, and orientation of defects may result in uncertainty in the analytical models (i.e., constitutive models) designed to assess structural allowable stresses and strains for the propellant. Defects that occur in propellant-liner interface may cause hot gas to be present near the wall of the rocket motor case. In addition, if a defect near the liner extends further into the propellant, the propellant may become detached from the bonding surface of the liner. Debonding may cause further defects (e.g., cracks) in the propellant, which may result in augmented and accelerated burning of the propellant, including near the wall of the case, as well as concerns regarding the structural impact of the decreased bonding with the liner.
In addition, environmental factors (e.g., moisture) may weaken the adhesion strength of the propellant binder to the surface of the reinforcing and combustible fillers in the solid rocket motor over time, which can result in reduced load bearing capability of the propellant. Because there is often uncertainty associated with the constitutive properties of the these polymeric systems especially when exposed to environmental aging, the term “health” of solid rocket motor is sometimes used to classify the launch readiness of the solid rocket motor and the propellant's ability to withstand damage during the dynamic launch event.
Conventional methods for screening the health of a solid rocket motor include radiographic (e.g., X-ray) inspection methods for verifying the health and quality of the propellant, liner, and insulation of a solid rocket motor. For example, FIG. 1 is a cut-out side view of a solid rocket motor 100 including a case 106 and propellant 108. Additional insulation materials may be located between the case 106 and the propellant 108. FIG. 1 shows the solid rocket motor 100 undergoing screening using conventional X-ray methods. For example, a first X-ray device 110 may be oriented such that the X-ray 112 may be transmitted substantially orthogonally to the solid rocket motor 100 to obtain an X-ray image through a thickness of the solid rocket motor 100 and into the propellant 108. As a result, defects in the propellant 108 may be detected. A second X-ray device 120 may be oriented at an angle such that the second X-ray device 120 obtains a tangential image of the solid rocket motor 100 to better detect defects in the additional insulation layers that largely go undetected by the first X-ray device 110.
Because of the size of solid rocket motors, this inspection method may require an undesirably large number of man-hours to obtain the images. In particular, the tangential image may be a very small field of view relative to the entire solid rocket motor 100 being imaged. As a result, after each tangential image is obtained, the solid rocket motor 100 may be rotated to a new position to obtain another tangential image. Each tangential image may require a substantial number of man hours to rotate the solid rocket motor 100 between each image, in addition to the time that is needed to expose the solid rocket motor 100 to an X-ray 122. Thus, although conventional inspection methods may have resulted in the detection of defects, the conventional inspection methods may be time consuming and costly.
Some additional conventional inspection approaches have attempted to detect voids using ultrasonic methods. These conventional ultrasonic methods have used either a repetitive high frequency (e.g., 1 MHz to 10 MHz) square or instantaneous pulse for the ultrasonic signal propagating into a material, and measuring and quantifying discreet reflections off of internal interfaces or potential defects. Images may be generated based on the peak amplitude responses from the ultrasonic signal. The conventional inspection methods, however, have not adequately provided the necessary depth to obtain accurate information any significant distance past the inside wall of case 106. Therefore, defects in the insulation materials may be undetected.